1. Field of the Invention
The present invention relates to a novel process for producing a composite molded product comprising a rigid part and a soft part. More particularly, the present invention pertains to a process for producing a composite molded product efficiently by a fusion bonding means, the product having a part made of an engineering plastic material which is rigid and superior in various properties, for example, a polycarbonate, and a part made of a soft thermoplastic elastomer.
2. Description of the Prior Art
Engineering plastics, which have excellent mechanical strength, are employed to produce mechanical parts or structural parts which are used under load. Various kinds of engineering thermoplastics are known, namely, polyamides, polyacetals, polycarbonates, modified polyphenylene oxides, polybutylene terephthalate, polysulfone, total aromatic polyester, etc.
In the meantime molded articles formed from thermoplastic elastomers (TPE) are also produced by injection molding technique which gives particularly high productivity and used in a variety of applications. Examples of known thermoplastic elastomers of this type include styrene-butadiene elastomers, polyolefin elastomers, polyurethane elastomers, vinyl chloride elastomers and acrylic ester elastomers.
The injection molding process is capable of effectively forming molded articles having complex shapes and suitable for mass-production, and it is applied to the abovementioned engineering plastics to efficiently produce various kinds of mechanical and structural parts.
There have recently been strong demands for increases in the level of performance and function of parts and members made of synthetic resin (plastic) materials. Under these circumstances, attempts have been made to combine together engineering plastics and thermoplastic elastomers, such as those mentioned above, to thereby form composite products. For this purpose, it is the most effective process to fusion-bond together these two different kinds of material into composite products by the injection molding technique that is a molding means common to the two different kinds of material.
However, the fusion bond properties of thermoplastic engineering plastics with respect to thermoplastic resins are, generally, not always good. The fusion bond properties with respect to thermoplastic elastomers (TPE) having high rubber elasticity are particularly inferior, so that these two materials cannot rigidly be bonded together. For this reason, when thermoplastic engineering plastics and thermoplastic elastomers (TPE) are to be combined together to produce composite products of high added value, e.g., composite parts and composite members, it is general practice to adopt a method wherein locking portions, for example, a combination of a recess and a projection, are provided at the joint of the two different kinds of material to thereby bond them together mechanically, or a method wherein an adhesive is applied to the joint of the two materials to bond them together.
The above-described conventional technique of forming a composite product from a thermoplastic engineering plastic and a thermoplastic elastomer (TPE) suffers, however, from the problems that the process is inefficient and the strength and sealing at the bond area are unsatisfactory.
More specifically, the method in which the two different kinds of material are mechanically bonded together through locking portions, for example, a combination of a recess and a projection, has the problems that the structure of the mold employed is complicated and the number of steps required for the process increases depending upon the structure. In view of the final shape of the composite molded product, it may be impossible to attain a strong composite structure. In addition, the mechanical bonding is incapable of attaining satisfactory waterproofness in the case of a composite molded product that is required to be waterproof, for example, water goggles in which the body portion is made of a rigid engineering plastic material and the edge portions that come into contact with the user's face are made of a soft and high-elasticity thermoplastic elastomer.
The method that uses an adhesive requires additional steps, for example, application of an adhesive, and suffers from the problem that the bond strength is low due to deterioration with time of the adhesive.